JP3506195B2 - Continuous casting method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous steel casting method capable of suppressing the occurrence of unsteady bulging.

[0002]

2. Description of the Related Art In continuous casting of steel, molten steel in a ladle is poured into a water-cooled mold through a tundish, and 1
The slab that has been cooled to form a solidified shell is continuously drawn out by the pinch rolls along the guide rolls, and further cooled by secondary cooling with water spray or mist spray between the guide rolls to completely solidify. To produce a cast slab.

Thus, until the slab is completely solidified, only the surface layer portion is solidified and the inside is unsolidified molten steel, and the solidified slab of the slab is guided by the static pressure of the molten steel as shown in FIG. A bulge (hereinafter referred to as bulging) has occurred between the guide roll and the guide roll. This bulging depends on conditions such as the drawing speed of the slab, the cooling rate, the steel type, the pitch of the guide rolls, etc., and becomes severe when the solidified shell is thin, the strength is small, and the guide roll pitch is large.

Normally, the bulging amount is a constant value without changing with time under constant casting conditions. The bulging in this case is called steady bulging, and there is no particular problem if the bulging amount is below a certain limit. However, if the steady bulging amount exceeds a certain limit, the casting speed will fluctuate due to the pulling resistance of the slab, etc., and the bulging amount will also fluctuate due to that effect. Becomes a state of unsteady bulging that fluctuates periodically.

When unsteady bulging occurs, the volume in the solidified shell fluctuates, which causes a periodic fluctuation of the molten metal surface in the mold. The amount of fluctuation of the molten metal level in the mold due to unsteady bulging may reach 40 to 50 mm, which causes abnormal solidification shell formation, deterioration of cast slab surface properties due to powder entrainment, deterioration of internal quality, cross-section cracks, white bands, etc. There are problems that defects occur, the casting speed must be greatly reduced, and productivity is also reduced.

[0006] Conventionally, as measures for preventing or reducing unsteady bulging, changes in guide roll pitch, improvement of cooling strength, improvement of precision of molten metal level control and slab drawing speed control, etc. have been carried out. It has not been resolved. In Japanese Patent Laid-Open No. 63-104767, the amount of bulging between guide rolls and the position of the molten metal surface in the mold are detected, and the pinch roll withdrawal speed and the molten metal injection speed into the mold are detected based on this output signal. A technique for adjusting and controlling the amount of bulging and controlling the position of the molten metal in the mold is disclosed. Further, Japanese Patent Application Laid-Open No. 1-166873 also discloses a technique of constantly measuring the amount of bulging between the guide rolls and controlling the casting speed by the measured value to adjust the variation amount of the mold surface.

[0007]

Problems to be Solved by the Invention JP-A-63-1047
In Japanese Patent No. 67,671, the difference between the preset value of the molten metal surface position and the bulging amount and the value actually detected is calculated, and the stopper driving device and the pinch are arranged to eliminate the difference. It outputs a signal to the roll driving device to adjust the molten steel pouring speed and the slab withdrawing speed, and controls the molten steel surface position and bulging amount. " Feedback control is performed so as to make the deviation from the value zero, but there is no description of a specific control method for realizing it.

Further, in JP-A-1-166873,
In the example, “V (casting speed) is changed so that α (pinch roll rotation speed fluctuation amount) is reduced at this time, but α is related to many other casting conditions, The most effective method of changing V is to shift the phase synchronized with the period of unsteady bulging by the time delay of the control system.
If the method of changing V by (unsteady bulging amount) is determined, δ can be automatically reduced by the control device. However, there is no description of a concrete method for changing the casting speed.

Generally, a method of increasing the drawing speed when the bulging amount is large and decreasing the drawing speed when the bulging amount is small to reduce the unsteady bulging can be considered. However, when the level of the molten metal decreases, that is, when the unsteady bulging amount is large, the drawing speed is decreased and when the molten metal rises, it is not adopted as a common sense. That is, when the unsteady bulging amount is small, a method of increasing the drawing speed is adopted. However, such a method cannot suppress unsteady bulging itself and cannot suppress fluctuations in the molten metal level.

An object of the present invention is to solve the above problems and to provide a specific method for suppressing unsteady bulging.

[0011]

Means for Solving the Problems Trans. ISI
J. vol. 26 (1986), P891, Analy
sis of bulging in Continuu
ously CastSlabs by Bendin
g Theory of Continuous Be
The paper am (Analysis of bulging of continuously cast slab by bending theory of continuous beam) examines influence of casting speed on unsteady bulging by numerical analysis considering creep deformation.

According to this, unsteady bulging occurs due to the fluctuation of the casting speed of ± 5%, the bulging amount is large when the casting speed is low, and the bulging amount is small when the casting speed is increased, or conversely is dented. It has been calculated that the value of the bulging amount becomes negative. This is due to the creep deformation of the solidified shell, and the present invention proposes a method of suppressing the fluctuation of the molten metal surface level with reference to this finding, suppressing unsteady bulging, and eventually stabilizing the molten metal surface level. .

[0013] That is, the present invention provides a continuous casting of steel, non-occurrence of steady bulging senses, is varied to thereby periodically tune the pulling rate to the period of unsteady bulging of the slab based on the detected knowledge, the Fluctuations in the withdrawal speed are
Continuous casting machine roll based on the temporal change of the amount of rusting
Increase the extraction speed when the unsteady bulging amount is large
However, when the unsteady bulging amount is small or a negative value (reverse
The present invention provides a continuous casting method characterized in that fluctuations are made to slow down the drawing speed when (when it is dented) and unsteady bulging is prevented.

Further, it is preferable to change the molten steel injection flow rate into the mold so as to offset the fluctuation level of the molten metal level in the mold caused by the fluctuation of the drawing speed. Further, the detection of occurrence of the unsteady bulging may be detected by a range finder or a displacement meter installed between rolls, and further caused by a fluctuation of the unsteady bulging amount detected by the range finder or the displacement meter. By varying the molten steel injection flow rate into the mold so as to cancel the variation in the molten metal level in the mold, fluctuations in the molten metal level can be eliminated .

It is also possible to estimate the temporal change of the unsteady bulging amount from the temporal change of the molten metal level in the mold.

[0016]

According to the present invention, when unsteady bulging occurs, the drawing speed of the slab is synchronized with the cycle of unsteady bulging to increase the drawing speed when the unsteady bulging amount is large, and the unsteady bulging amount is small. At times, the drawing speed is periodically changed so as to slow it down. That is, it is possible to suppress the amount of unsteady bulging by changing the time for the solidified slab of slab to stay between the guide rolls and changing the amount of creep deformation.

Further, when the casting speed is increased, the molten metal injection amount is increased and the casting speed is reduced so as to cancel the fluctuation of the molten metal level in the mold caused by the fluctuation of the casting speed for suppressing unsteady bulging. At times, the injection flow rate into the mold is periodically changed in synchronization with the change in casting speed so as to reduce the molten metal injection amount. The level change in the molten metal level in the mold does not increase, and the molten steel injection amount is changed according to the fluctuation in the unsteady bulging amount detected by the distance meter or displacement meter. You can almost eliminate it.

[0018]

FIG. 1 is a schematic diagram of a continuous casting apparatus and a control system for suppressing unsteady bulging showing an embodiment of the present invention.
The molten steel 1 in the tundish 2 is supplied into the mold 3 through the immersion nozzle 7. The solidified shell 8 is formed in the mold 2, guided by the guide roll 4, pulled out downward by the pinch roll 6, and cooled to form the slab 5. Level 1 in the mold
0 is detected by the level gauge 12 and input to the arithmetic unit 16. The speed of the pinch roll 6 is the drawing speed detector 1
5 is detected and input to the arithmetic unit 16. The bulging amount is detected by the bulging amount detector 13 and input to the arithmetic unit 16. The arithmetic unit 16 calculates this and outputs a necessary output to the sliding nozzle 11 and the pinch roll driving unit 14 to control the pouring amount and the drawing speed.

FIG. 2 is an explanatory diagram showing fluctuations in the molten metal level, the bulging amount, the drawing speed, and the sliding nozzle opening when unsteady bulging occurs and when the control according to the present invention is performed. When steady bulging occurs (0 to t in Fig. 2)
₁ between) is molten metal surface level, bulging amount, drawing speed, but sliding nozzle opening is stable, the unsteady bulging occurs (between t ₁ ~t ₂ in FIG. 2) molten metal surface level is periodically It fluctuates greatly. At this time, the fluctuation of the molten metal level is synchronized with the fluctuation of the bulging, and the maximum drawing speed is 2-3.
% Variability is observed. The bath level is a bath level meter 12
According to the value detected by the sliding nozzle 11
Although it is controlled using a flow rate regulator such as a stopper or a stopper, it is not possible to suppress fluctuations in the molten metal surface level due to unsteady bulging with a normal control method.

This is because there is a time delay of the control system (for example, a time delay of the sliding nozzle opening, Δt _{1 in} FIG. 2, etc.) with respect to a short cycle variation, and if the control gain is increased, it easily diverges. Ordinary PID for reasons such as
This is because control or the like cannot handle it. Therefore, in the present invention, when the occurrence of unsteady bulging is detected by the molten metal level gauge 12, the drawing speed is increased based on the output of the bulging amount detector 13 when the bulging amount is large, and the drawing speed is reduced when the bulging amount is small. As described above, the arithmetic device 16 outputs a signal to the pinch roll driving device 14 so as to be synchronized with the fluctuation of the bulging amount (t ₂ to t _{3 in} FIG. 2).
while). This suppresses unsteady bulging.
At this time, the change amount of the drawing speed is about ± 5% at the maximum, and it may be changed according to the magnitude of the bulging amount.

Further, in order to suppress the fluctuation of the molten metal level, the fluctuation amount ΔQ _v (k of the molten steel drawing amount corresponding to the fluctuation of the drawing speed
g / min) is ΔQ _v = ΔV _r × S × ρ where ΔV _{r is the} amount of fluctuation in the drawing speed (m / min) S is the cross-sectional area of the slab (m ² ) ρ is the molten steel density (kg / m ³ ). In order to offset this ΔQ _v , the sliding nozzle opening is changed in synchronization with the change in the drawing speed to change the molten steel injection amount. Further, the variation amount ΔQ _b (kg / min) of the molten steel drawing amount caused by the variation amount ΔV (m ³ / min) of the molten steel volume in the cast due to the bulging variation is ΔQ _b = ΔVρ = α (Δδ / Δt) ρ where α : Proportional constant δ: bulging amount (m) (Δδ / Δt): bulging fluctuation speed (m / min). In order to offset this ΔQ _b , the sliding nozzle opening is changed in synchronization with the bulging change to change the injection amount. As a result, the molten metal level can be kept constant. At this time, it is more effective to advance the opening timing of the sliding nozzle by a time delay (the time required for the molten steel to reach the inside of the mold from the sliding nozzle position, Δt _{2 in} FIG. ₂ ).

Although the method of detecting the unsteady bulging amount by the bulging detector 13 has been described above, the periodicity of the unsteady bulging during casting under a constant condition is very regular, and the hot water for several times of hot water is used. It is possible to estimate the temporal fluctuation timing of the unsteady bulging amount by measuring the temporal fluctuation of the surface level. Therefore, if unsteady bulging occurs, the casting speed of the present invention can be controlled to fluctuate periodically without directly measuring the amount of bulging. In this case, it is appropriate that the fluctuation amount of the speed is ± 1 to 2%, and it is preferable to intermittently perform this control method every few minutes. This makes it possible to confirm whether the unsteady bulging is suppressed.

The present invention uses ultra low carbon steel with a slab size of 1.2.
5m × 0.22m, casting speed 2.25m / min, applied to unsteady bulging that occurred during casting with a specific water amount of 2.5 liters / kg, and withdrawal speed synchronized with the bulging cycle 2.20m / min to 2 As a result of fluctuating sinusoidally within the range of 30 m / min and varying the sliding nozzle opening in synchronization with the bulging fluctuation, the level of molten metal decreased from ± 10 mm to ± 3 mm after application of this control, and 5 minutes After that, unsteady bulging disappeared and the surface level was ± 1 to 2 mm.
Decreased to.

[0024]

According to the present invention, the drawing speed is periodically changed in synchronization with the change of the bulging amount and the opening of the sliding nozzle is changed. It has become possible to suppress the fluctuations in the level of the molten metal. According to the present invention, there is no need to significantly reduce the casting speed, it is possible to prevent a decrease in productivity, and it is also possible to prevent deterioration of the quality of the cast slab.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a control system of the present invention.

FIG. 2 is an explanatory diagram showing a relationship between a state quantity and an operation quantity when unsteady bulging occurs.

FIG. 3 is an explanatory diagram of unsteady bulging.

[Explanation of symbols]

1 Molten Steel 2 Tundish 3 Mold 4 Guide Roll 5 Cast Piece 6 Pinch Roll 7 Immersion Nozzle 8 Solidification Shell 10 Mold Surface 11 Sliding Nozzle 12 Level Meter 13 Bulging Amount Detector 14 Pinch Roll Drive Device 15 Extraction Speed Detection Device 16 arithmetic unit

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ identification code FI B22D 11/20 B22D 11/20 A (58) Fields investigated (Int.Cl. ⁷ , DB name) B22D 11/16 104 B22D 11 / 18 B22D 11/20

Claims (5)

(57) [Claims] 1. In continuous casting of steel, the occurrence of unsteady bulging is detected, and based on the detection, the withdrawal speed of the slab is periodically varied in synchronization with the cycle of unsteady bulging, and the withdrawal speed Fluctuation is the time of unsteady bulging amount
Bulge between rolls of continuous casting machine
The unsteady balgine speeds up the extraction speed when the
If the pulling amount is small or is a negative value,
The continuous casting method is characterized by dynamic and preventing unsteady bulging. 2. Further, it is caused by the fluctuation of the drawing speed.
Melt in the mold so as to offset the fluctuation level in the mold level.
The continuous casting method according to claim 1, wherein the steel injection flow rate is varied . 3. The detection of the occurrence of the unsteady bulging is based on
Detecting with a distance meter or displacement meter installed between
Continuous casting method according to claim 1 or 2, wherein the at. 4. Further, it is detected by the distance meter or the displacement meter.
Level in Mold Caused by Variation of Unsteady Bulging Amount
Molten steel injection flow rate into the mold so as to cancel the level fluctuation amount
The method of continuous casting according to claim 3, wherein 5. A method for casting the change in the unsteady bulging amount with time.
The continuous casting method according to claim 1, wherein the casting level is estimated from the temporal change of the molten metal level in the mold .